This invention relates to a process for the preparation of foundry cores and molds.
Curable resins are widely used in the foundry industry for making molds and cores which predetermine or produce the dimensions and cavities of the castings. Of particular importance in this regard is hot-curing and cold-curing mold and core sand binders based on phenolic, furan, and amino resins which contribute considerably towards economic mass-production casting and towards improving quality. At present, energy costs are closely related to process costs. Accordingly, processes in which as much energy as possible is saved are gradually acquiring a priority position. To help satisfy the need to save energy and costs, even with large series, so-called cold-box processes have in recent years been increasingly significant for the production of cores. Based on the use of curable resins, these processes represent a generally new processing variant in which one of the components is introduced into the curing reaction as a gas or aerosol. In foundry parlance, the polyurethane blowing process is known as a cold-box process and has been introduced under this name into the German foundry industry. According to the process described in German Auslegeschriften 1,483,521 and 2,011,365, polyhydroxyl compounds and polyisocyanates are mixed with the core sand and then crosslinked with an amine-based blowing agent as catalyst, thereby forming a polyurethane. The preferred polyhydroxyl compounds used in this process are resols produced, for example, using cobalt or lead naphthenate as catalyst and are often referred to in the relevant literature as benzyl ether resins. These compounds must be free from water because water would react prematurely with the polyisocyanate. To adjust viscosity to 130 to 450 mPa.multidot.s, the phenol resols contain approximately 30 to 35% by weight high-boiling solvents, such as aromatic and aliphatic hydrocarbons, esters, ketones, and the like. The polyisocyanates used in this process are generally diphenylmethane-4,4'-diisocyanate or derivatives thereof which are also diluted with solvents of the above-mentioned type. The cold-box binders are generally formulated in such a way that they are mainly used in a ratio of 1:1 with polyisocyanate. The total quantity of binder (including solvent) is about 1 to 2% by weight, whereas the total quantity of catalyst is about 0.02 to 0.10% by weight (based in both cases on the amount of sand). The process is carried out by initially shooting the core sand into the core box and then blowing in a mixture of amine and air as a gas or aerosol. The amines used in the process are generally triethylamine, dimethylethylamine, or dimethylisopropylamine which are introduced into the core box under a pressure of 0.2 to 2 bar. The residual gases are removed from the core with heated purging air or carbon dioxide gas and may be treated in an acid scrubber charged with dilute sulfuric acid or phosphoric acid. Suitable scrubbers work on the countercurrent principle. Significant advantages of this process include a considerable increase in productivity, smooth core surfaces by virtue of the excellent flowability of the sand during shooting, high dimensional accuracy through cold curing, and high strengths of the mold materials despite extremely short curing times. A disadvantage of this process is the limited storage life of the core sand because, even in the absence of amine, certain polyurethane polyaddition reactions begin in the core sand. In addition, the solvents of the binder and polyisocyanate solutions partly evaporate at the surface. Consequently, a reduction in strength occurs if the sand is processed a few hours after preparation. Thus, the cold flexural strength measures approximately 5.5 N/mm when blowing is carried out immediately after preparation of the sand but only 4 N/mm when blowing is carried out one hour after preparation of the sand. Reductions in strength of 25% and 60% are observed after 2 hours and 3 hours, respectively. In addition, the sand is impossible to process about 8 hours after preparation because it has solidified as the polyurethane reaction proceeds.
Accordingly, the problem addressed by the present invention was to develop a casting resin mixture which gives improved storage life for the prepared core sand without a reduction in the strength after a few hours. An extended processing time such as provided by the present invention affords considerable advantage in the production process used for the core sands and leads to a significant reduction in production costs because the core sand needs to be prepared with the polyurethane raw materials only once a day or less. Another problem addressed by the present invention was to develop core sand binders having higher strength values and higher heat resistance levels, which are needed in particular for more recently developed foundry applications.